1. Field of the Invention
The present invention relates to a digital communication system which is not influenced by multipath interference and applicable to, e.g., mobile radio communication.
2. Description of the Prior Art
In a conventional digital communication system such as a mobile radio communication system, a digital signal modulated by a binary information sequence is received through different propagation paths whereby the bit error rate is extremely deteriorated by mutual interference of the multiple waves. Such a disadvantage is now described with respect to a conventional binary phase shift keying (BPSK) signal. In FIG. 1, symbol (A) denotes a binary information sequence consisting of symbols 1 and 0 and symbol (B) denotes phase change of a BPSK signal obtained in correspondence to the binary information sequence. When differential detection is performed, the phase of the BPSK signal is shifted by +.pi. radians with respect to the binary information symbol 1 and no such change is caused with respect to 0.
FIG. 2 shows a differential detection circuit which demodulates the BPSK signal to detect the original binary information sequence. The BPSK signal applied to an IN terminal 1 is divided into two signals to be fed to one input terminal of a multiplier 2 as well as to the other input terminal thereof through a delay circuit 3 having a delay time of one time slot T. Then the two signals are multiplied by the multiplier 2 to be subjected to removal of frequency components twice the carrier frequency by a low pass filter (LPF) 4, and the original binary information signal, which is outputted from an OUT terminal 5, is reproduced.
For convenience of description a two ray model is assumed hereinafter as an example of multipath propagation.
In the case where the BPSK signal applied to the IN terminal 1 is the superposition of a BPSK wave (desired wave--hereinafter referred to as D wave) and its replica (undesired wave--hereinafter referred to as U wave) delayed by .tau. from the D wave, the output at the OUT terminal 5 is extremely reduced around portions in which the relative amplitude ratio .rho. of the U wave to the D wave is 1 and the carrier phase difference .phi. between the D and U waves is .pi. radians whereby the bit error rate is extremely inferior.
FIG. 3 illustrates the time relation between the D and U waves. Symbol T denotes the length of a time slot for transmitting a binary information symbol. Symbol a denotes a section from the beginning of the D time slot to the beginning of the U time slot, symbol b denotes a section from the beginning of the U time slot to the midst of D time slot, symbol c denotes a section from the midst of D time slot to the midst of U time slot, and symbol d denotes a section from the midst of U time slot to the end of the D time slot.
With respect to demodulation outputs in the respective sections a to d, at the OUT terminal 5 in FIG. 2, the output in the section a depends on the symbol of a preceding bit (not shown), whereas the outputs in the sections b, c and d are not influenced by symbols of preceding bits but determined by the symbol of the present bit.
The respective outputs e(t) in the sections b, c and d are obtained by the following expressions (1):
When the symbol is 1: EQU e(t)=(1+.rho..sup.2 +2.rho.cos.phi.)
When the symbol is 0: EQU e(t)=-(1+.rho..sup.2 +2.rho.cos.phi.) (1)
As obvious from the expressions (1), the respective outputs e(t) in the sections b, c and d are substantially reduced nearly to zero when the relative amplitude ratio .rho. of the U wave to the D wave tends to 1 and the carrier phase difference .phi. between the D and U waves tends to .pi. radians, whereby the so-called eye pattern may be dominated by the section a, and eye degradation and abrupt timing fluctuation occur. In general, the eye is deteriorated and burst errors occur, and consequently the bit error rate is deteriorated in portions around the condition of severe delay distortion (.rho.=1 and .phi.=.pi. for two-wave model) in multipath propagation.